1. Field of the Invention
This invention relates to an improved method for the continuous production of stable aqueous ammonium polyphosphate compositions.
2. Description of the Prior Art
Various methods for the continuous production of stable aqueous ammonium polyphosphate compositions by the reaction of ammonia and wet-process superphosphoric acid are well known in the art. A most effective technique is disclosed in Mullen, U.S. Pat. No. 3,459,499 (1969), whose discussion of the state of the art is incorporated herein by reference. Mullen's method is a continuous process for forming an aqueous ammonium phosphate composition stabilized by the presence of polyphosphate ions, comprising the steps of continuously passing through an endless path preformed ammonium phosphate composition containing polyphosphate ions, continuously adding to this stream at different points predetermined amounts of first the aqueous ammonia and then at a point downstream superphosphoric acid containing a sufficient amount of polyphosphate ions to stabilize the product and creating a turbulence in the stream, whereby the aqueous ammonia and acid react to form a new quantity of the aqueous ammonium phosphate composition, and withdrawing downstream from the points of addition of the aqueous ammonia and superphosphoric acid a portion of the aqueous ammonium phosphate composition.
Being performed in a closed system with control of environmental conditions and constituent characteristics, Mullen's method minimized the prior art problems of the loss of ammonia by vaporization, the loss of polyphosphate forms by hydrolysis, and the formation of the less soluble mono-ammonium and di-ammonium phosphates. Further, Mullen's method provided temperature control within the system and maintenance of suitable concentrations of constituents and properly mixed pH controlled neutralizations.
However, Mullen's method required a complex mixing system for adding the ammonia and superphosphoric acid to the stream of preformed ammonium phosphate composition at different points, with the ammonia being added to the stream in advance of the superphosphoric acid.
An additional problem is presented by the expense and difficulty of obtaining high conversion wet-process superphosphoric acid, that is, acid containing sufficient polyphosphate to sequester metal impurities in the final aqueous ammonium polyphosphate compositions and to produce a stable composition. Thus, in addition to reacting superphosphoric acid with ammonia and preserving the polyphosphate initially present in the acid feed, it is necessary to convert a portion of the orthophosphate to polyphosphate so that sufficient polyphosphate is available to sequester any metal impurities which may be present in the finished composition.
Pipe reactors are a well-known means for converting orthophosphate to polyphosphate. Wet-process superphosphoric acid and ammonia are introduced into the pipe reactor for reaction therein, and a reaction mixture containing the desired amount of polyphosphate is discharged from the pipe reactor.
However, there are problems associated with the use of pipe reactors in prior art methods for the continuous manufacture of stable aqueous ammonium polyphosphate compositions. Achieving the desired extent of conversion of orthophosphate to polyphosphate within the short residence time of the reaction mixture in the pipe reactor generally requires either the addition of external heat or the generation of sufficient heat by the exothermic reaction between the superphosphoric acid and ammonia. The latter method additionally necessitates efficient mixing of the acid and ammonia within the pipe reactor. Both methods are problematical because pipe reactors are subject to the severe corrosive effects of superphosphoric acid at elevated temperatures. Further, recovery of the product in its desired finished form often necessitates treatment of the reaction mixture subsequent to its discharge from the pipe reactor. Such treatment often involves the addition of reactant or the separation of unreacted ammonia and other volatiles from the ammonium polyphosphate. The prior art methods have taken a large number of approaches to solve these problems.
For example, Kearns, U.S. Pat. No. 3,464,808 (1969) discloses a method wherein an ammonia stream is introduced into the center of an outer concentric stream of superphosphoric acid in a pipe reactor. In order to prevent a buildup of solids in the pipe reactor, a centrifugal motion is imparted to the acid prior to the contact with ammonia so that the acid surrounds the ammonia stream. Stream and other gases are permitted to escape when the melt product is quenched by being impelled through a gaseous medium upon exiting from the pipe reactor. Additional ammonia is added to neutralize the melt after it is dissolved in an aqueous solution. Alternately, quenching of the melt can be effected by feeding the melt directly into the aqueous solution. However, the second method of quenching requires more cooling than the first method because the steam produced by the conversion of orthophosphate to polyphosphate recondenses in the cooled solution.
A modification of this approach is found in the patent to Kearns U.S. Pat. No. 3,695,835 (1972) which disclosed a method wherein mixing is effected in the same manner as in U.S. Pat. No. 3,464,808. However, after mixing, ammonium phosphate is formed in one step, and then the ammonium phosphate is subjected to hot gases in a second separate step to raise the temperature whereby a portion of the ammonium phosphate is converted to ammonium polyphosphate. Quenching of the ammonium polyphosphate melt is effected as in U.S. Pat. No. 3,464,808.
Legal, U.S. Pat. No. 3,503,706 (1970) disclosed another method wherein ammonia is introduced into a stream of superphosphoric acid in a pipe reactor, and a melt is produced. Free water and gaseous phase are separated from the melt by passing the melt from the pipe reactor into a chamber wherein the melt is agitated and dissolved. Then additional ammonia is added to the solution to adjust its pH and ammonia content to the level desired in the ammonium polyphosphate composition.
Meline, U.S. Pat. No. 3,775,534 (1973) was filed Nov. 15, 1971 and was assigned to the Tennessee Valley Authority. This patent discloses a method for producing liquid fertilizer solutions whereby wet-process superphosphoric acid and ammonia are combined in a pipe reactor and the resulting melt is fed to a solution reactor vessel or reaction tank containing the final desired liquid fertilizer solution. Neutralization of a portion of the acid and conversion of a portion of the orthophosphate to polyphosphate occur in the pipe reactor. The conversion reaction is quenched when the hot melt exiting from the pipe reaction is introduced into the cooler liquid fertilizer solution in the solution reactor vessel. The neutralization is completed in the solution reactor vessel. The method teaches that additional ammonia is introduced into the liquid fertilizer solution in the solution reactor vessel, if less than one hundred percent of that required is fed to the pipe reactor. Alternately, the melt can be quenched by cooling it to about ambient temperature whereby the melt is solidified into a friable solid. The soluble reactor vessel is apparently open to the atmosphere with the resulting danger of losses due to evaporation. The patent does not disclose a way of avoiding corrosion of the pipe reactor or the manner in which the acid and ammonia are combined in the pipe reactor.
The publication "Use of a Pipe Reactor in Production of Liquid Fertilizers with Very High Polyphosphate Content" in Solutions, March-April, 1972, pp. 32-45, by R. S. Meline, R. G. Lee, and W. C. Scott, all of the Tennessee Valley Authority discloses a method wherein an ammonia stream is introduced into the center of an outer concentric stream of superphosphoric acid in a pipe reactor and the melt produced is passed from the pipe reactor to a conventional liquid fertilizer reaction tank where it is dissolved in an aqueous solution. Water and additional ammonia are added to the solution in the tank, and the solution is stirred and cooled. The authors reported that some of the polyphosphate in the melt was lost during dissolution in the reaction tank because of hydrolysis, unless the temperature of the liquid fertilizer was kept below 150.degree.F. in the tank. The authors pointed out that an insoluble scale formed in the pipe reactor and eventually plugged the pipe reactor and that a means for controlling its rate of formation was needed. They indicated that cooling the pipe reactor by water jacketing might substantially reduce the rate of scale formation.
The report "New Developments in Fertilizer Technology" presented by the Tennessee Valley Authority on Oct. 17-18, 1972 at the National Fertilizer Development Center in Muscle Shoals, Ala., discloses a method wherein an ammonia stream is introduced into the center of an outer concentric stream of superphosphoric acid in a pipe reactor and the melt produced is passed from the pipe reactor to a recycle stream of the desired final liquid fertilizer. The report indicated that when all of the ammonia to be added was fed to the pipe reactor, there was a decrease in the polyphosphate content of the product. Consequently, only a portion of the ammonia required is fed to the pipe reactor, and the remainder is fed to the recycle stream downstream from the pipe reactor. Water is also added to the recycle stream at this point. Use of a water jacket to cool the walls of the pipe reactor prevented corrosion and retarded the buildup of scale on the walls of the pipe reactor. However, the report indicated that cooling the pipe reactor caused a decrease in the polyphosphate content of the product.
Groenveld, U.S. Pat. No. 3,730,700 (1973) discloses a method wherein a stream of wet-process superphosphoric acid is introduced into the center of a stream of ammonia in a pipe reactor at carefully controlled linear velocities and the streams are accelerated through a first reactor zone of the reactor. The velocity of the ammonia, calculated at its point of entrance to the reactor, must be more than 100 feet per second, otherwise solid deposits form on the walls of the pipe reactor. On leaving the pipe reactor, the reaction mixture is discharged through a restricted opening, and the pressure is reduced so that the gaseous impurities and moisture present in the reaction mixture are removed by flash evaporation.
Meline et al. U.S. Pat. No. 3,733,191 (1973) discloses a method wherein ammoniation of wet-process superphosphoric acid occurs in two reactors, only the second of which is a pipe reactor. The ammonia is introduced into an acidic medium in the pipe reactor, and the product exiting from the pipe reactor, after a relatively long residence time of from 15 seconds to 1 minute, is fed into a foam disengager which is maintained under negative pressure and wherein foam and water vapor are separated from the melt product.
Finally, Burns, U.S. Pat. No. 3,734,708 (1973) discloses a method wherein an ammonia stream moving at a velocity of at least 500 feet per second and an acid stream are introduced into a mixing zone from opposite directions, the mixed acid and ammonia are conveyed through a U-shaped reaction zone for from 0.10 to 0.18 seconds, steam is removed from the ammonium polyphosphate product, and air is drawn through the steam-free product to cool it. Additionally, in Burns' method it may be necessary to introduce ammonia into the product to adjust its pH and ammonia content to the level desired in the ammonium polyphosphate composition.
Contrary to prior art methods, it has been found that corrosion of the pipe reactor can be minimized without the use of a water-jacket to cool the pipe reactor as in the Tennessee Valley Authority report and without imparting a centrifugal motion to the acid as in the Kearns patents, but instead by the introduction of a stream of the acid into the center of a stream of ammonia and away from the walls of the pipe reactor. Groenveld discloses a similar method of introducing the reactants but requires that the streams of acids and ammonia be accelerated as they flow through the pipe reactor. On the contrary, in the method of this invention, the velocity of the combined streams of the acid and ammonia in the pipe reactor is reduced in order to achieve efficient mixing of the streams within the pipe reactor. The amounts of the acid and ammonia required in the analysis desired for the final ammonium polyphosphate composition are added in a single step and by means of a simple flow system, and the reactions are completed within a few seconds after the reactants are introduced into the pipe reactor. Further, this method has the advantages of the method of Mullen, U.S. Pat. No. 3,459,499 (1969). It employs a closed system and provides temperature control and maintenance of suitable concentrations of constituents and properly mixed pH controlled neutralizations. It does not employ a tank reactor. No losses of ammonia by volatilization or of polyphosphate by hydrolysis occur, and no separations are required.